Pipeline pigs are of various designs and are used for a variety of purposes. One type of pig is inserted into pipelines and pushed through pipelines to clean the pipeline. Other pigs may be used for batching or separating different products that are being transported in the pipeline by sealing to the walls of the pipeline and moving along the pipeline with the products. Still others are used to inspect the pipeline.
In certain pipelines, pumping stations are located at intervals along the line, and efficient pipeline design calls for a smaller diameter pipe connected to the output of the pumping station and a larger diameter pipe connected to the suction of the pumping station. Thus when moving downstream from one pumping station to the next, at a mid-point between the two pumping stations the pipeline transitions from, for example, a 36″ diameter pipe to a 48″ diameter pipe.
In order to clean these dual diameter pipelines, dual diameter pigs have been developed that have an expanded diameter suitable for operations to clean the larger diameter pipe, and a compressed diameter suitable for operations to clean the smaller diameter pipe. When the pipeline transitions from the smaller to the larger diameter pipe, the pig automatically expands. Various means for providing such expandable pigs are known, for example as disclosed in U.S. Pat. No. 5,457,841 to Minton. The dual diameter pigs are launched into the smaller pipe portion of the pipeline at a pig trap located at a pumping station.
At the pumping station the pipeline comes out of the ground and an access port is provided to allow the pig to be pushed into the pipeline. A diverter conduit and valves are provided such that product can continue to flow through the pipeline while the pig trap is isolated from the flow and drained. Once drained a door covering the port is opened and the pig is pushed into the trap, the door is closed and valves are operated to direct pipeline flow through the pig trap and thus carry the pig down the pipeline.
Since the pig is launched into the smaller pipe at the output of the pumping station, it must be compressed to its smaller size. The pig trap will have a diameter equal to the diameter of the smaller pipe. For example with dual diameter pipeline having a 36″ smaller pipe and a 48″ larger pipe, the pig will have corresponding compressed and expanded diameters, and the pig trap will have a 36″ diameter. A conical reducer is typically used to compress the pig for insertion into the trap. The reducer will have a smaller end with a 36″ diameter opening fixed to the 36″ pig trap opening, and tapering outward to a larger open end with about a 48″ opening. The pig is placed into the 48″ opening and forced through the reducer into the 36″ pig trap. Such a pig launching operation is generally described in U.S. Pat. No. 5,296,039 to Cooper and U.S. Pat. No. 5,769,955 to Kozisek.
Such pigs for this size of pipeline are quite large, typically eight feet or more in length, and are quite heavy as well, making them awkward to maneuver into the trap, and requiring considerable force to push through the reducer. The pigs typically have a solid core against which a force can be exerted to force the pig through the reducer into the trap. The is operation is complicated by the fact that once the pig trap is open, petroleum fumes are released and pose a fire hazard which require that operations include appropriate safety precautions. Often pigs are maneuvered into place by a loader or backhoe vehicle, and then pushed through the reducer into the trap by the vehicle as well, however operating internal combustion vehicles in proximity to the open pig trap presents a fire hazard. Alternatively it is known to rig a hydraulic cylinder to push the pig into the trap, however because of the length of the pig, such hydraulic cylinders must have a very long stroke of 10-12 feet, and so are costly and awkward to position and transport.